1. Field of the Invention
The present invention relates to a relay receiver for performing a heterodyne relay to be used for a radio communication system.
2. Description of the Related Art
Radio communication systems using microwaves, for example, are widely used for transmission of telephone, television and other signals. The field of digital communication technology has recently been developing and the role of radio communication systems has become increasingly important. The Integrated Services Digital Network, or ISDN, is a good example.
Radio communication systems require a relay system to perform long-distance communication. These systems generally use a heterodyne relay, a detection relay or a direct relay.
The heterodyne relay system converts a received microwave into an intermediate frequency band wave and then reconverts the intermediate frequency band wave into a microwave for transmission.
The detection relay system demodulates the received microwave to provide a base band signal and then remodulates the base band signal into a microwave for transmission.
The direct relay system amplifies the received microwave, for example, in the frequency band of the received microwave.
The system most used is the heterodyne relay system because modulation distortion is not added every time the relay is conducted.
FIG. 1 shows the general structure of such a heterodyne relay system. A relay station receives n channel communication signals whose receiving frequencies differ from each other by 80 MHz, performs a heterodyne relay, and thereafter transmits n channel communication signals whose frequencies also respectively differ from each other by 80 MHz.
Note that the communication signal transmitted from the upper station includes n channels with central frequencies from f.sub.1 MHz to (f.sub.1 +80(n-1)) MHz every 80 MHz.
The above communication signals corresponding to the respective channels are received by a common receiving antenna, are divided into several parts by a wave divider (not shown), and are input to the relay receiving apparatus. In FIG. 1, only the relay receiver corresponding to a received signal with a central frequency f.sub.1 is shown, but the same structure may be applied to the other central frequencies. This relay receiver comprises a receiving frequency converting portion 1, intermediate frequency amplifier portion 2 and bandpass filter 3.
The receiving frequency converting portion 1 converts the frequency of the received signal with central frequency f.sub.1 transmitted from the upper station into an intermediate signal. The converted output is amplified to reach a predetermined level by intermediate frequency amplifying portion 2, which includes an automatic gain control unit. The amplified output is limited to a predetermined band by bandpass filter 3 to be transmitted to a relay transmitter. The bandpass filter performs waveforming and noise removing functions. A relay transmitter comprises transmitting frequency converting unit 41 and transmitting high frequency amplifier 42.
The relay transmitter converts the output of the bandpass filter 3 into transmission frequency f.sub.2 by means of a transmission frequency converting unit 41. This converted output is then amplified by a transmission high frequency amplifier 42 to reach a predetermined level. This amplified output is finally mixed with the transmission signal of the other frequency bands through a wave divider (not shown), and is transmitted to the lower stage through a transmission antenna.
The operation of the relay receiver and relay transmitter recited above is similarly applied to the communication signals corresponding to the other central frequencies (f.sub.1 +80) MHz to (f.sub.1 +80(n-1)) MHz.
In these systems, fading, caused, for example, by rain, may occur in the transmission path, reducing the receiving level of the received signal below a predetermined level. When transmitting a digital signal, it is thus necessary to prevent an increase in the bit error ratio of received signal channels and to reduce interference between adjacent channels as much as possible.
FIG. 2 shows a conventional relay receiver of FIG. 1. The receiving frequency converting portion 1 in FIG. 1 comprises units 11 to 14 of FIG. 2, and the intermediate frequency amplifying portion 2 in FIG. 1 comprises units 21 to 28 of FIG. 2.
Here, the transmission signal is a digital modulation wave having a band width equal to clock frequency f.sub.CK (which is called f.sub.CK hereinafter).
In FIG. 2, a received signal with a central frequency f.sub.1 and a standard arriving level of minus 20 dBm is received, for example, through the receiving antenna, amplified by a receiving high frequency amplifier 11, and then inputted to frequency converter 13 through band pass filter 12 with a band width of twice f.sub.CK. This band pass filter 12 performs a waveforming and a noise removing function.
In addition a receiving station signal is inputted to the converter 13 by receiving station oscillator 14. Thus, the input receiving signal is then converted into an intermediate frequency f.sub.IF, and this converted output is amplified by intermediate frequency amplifier 21.
The amplified output is passed through a variable attenuator 22 and is further amplified by intermediate frequency amplifier 23. A portion of the output from intermediate frequency amplifier 23 is detected by detector 25. The detected voltage outputted from detector 25 is applied to comparator 24, which compares the above detected voltage with a standard voltage V.sub.0. The amount of attenuation performed by variable attenuator 22 is controlled based upon a gain control signal outputted from the comparator so that the difference between the detected voltage and the standard voltage V.sub.0 becomes 0.
Therefore, the output level from intermediate frequency amplifier 23 is kept constant even if the receiving level of the received signal varies.
The output of intermediate frequency amplifier 23 is inputted to intermediate frequency band pass filter 26, which limits the band width of the input signal to the band width 1.5 times f.sub.CK. Intermediate frequency band pass filter 26 performs the function of waveformation and a noise removal.
The output of this intermediate frequency band pass filter 26 is next transmitted to relay equalizer 27, which equalizes the relay distortion caused by high frequency band pass filter 12 and intermediate frequency band pass filter 26. The output of the relay equalizer 27 is then further amplified by intermediate frequency amplifier 28.
As is described above, the amplification operation is conducted by a relay receiver. In a system with this structure, if the receiving level of the received signal with a central frequency f.sub.1 is at a standard receiving level, the frequency characteristic of the amplified signal outputted from the relay receiver becomes as shown in FIG. 3A. The frequency characteristics of a transmitted signal with a central frequency f.sub.2 outputted from the relay receiver has the same characteristics obtained by replacing the central frequency f.sub.IF in FIG. 3A with f.sub.2. Therefore, the transmitted signal has the band width of 50 MHz.
If the receiving level of the received signal decreases to a level of about 70 dBm, for example, because of rain or an occurrence of fading along its transmission path, the output of the intermediate frequency amplifier 23 in FIG. 2 is decreased in a corresponding manner. In this case, comparator 24 is controlled so that the amount of attenuation performed by variable attenuator 24 becomes almost 0 in accordance with a gain control signal.
As a result of the above operation, the total gain of intermediate frequency amplifier 21, variable attenuator 22, and intermediate frequency amplifier 23 increases and the output of intermediate frequency amplifier 23 is kept constant even if the receiving level changes.
However, the increase in gain causes the noise component passing through intermediate frequency band pass filter 26 to increase. The noise component, which has been either inputted to receiving high frequency amplifier 11 or produced by receiving high frequency amplifier 11, is superimposed on the received signal to relatively high level, and thus the noise component superimposed on the received signal cannot be disregarded. The resulting frequency characteristic of the amplified signal outputted from the relay receiver is shown in FIG. 3B. Thus, the frequency characteristic of the transmitted signal outputted from the relay receiver is equal to the characteristic obtained by replacing the central frequency f.sub.IF in FIG. 3B with f.sub.2.
As should be clear from the above explanation, the frequency characteristics of the transmission signal include the noise component designated by a slanted line in FIG. 3B, in addition to the real transmission signal component having a band width of plus or minus 25 MHz with respect to the central frequency. In this case the noise component ranges from plus or minus 40 MHz to plus or minus 50 MHz with respect to the central frequency, thereby substantially increasing the band width of the transmission signal.
In the case where the receiving level of the received signal is low, S/N (Signal vs. noise ratio) of the transmitted signal decreases. Thus, during transmission of a digital signal, for example, the bit error ratio in a channel increases when the transmission signal having the above frequency characteristic is transmitted from the relay receiver to the local station (not shown). This transmission signal is then demodulated in the local station.
The band width of the transmission signal is widened in the frequency range from plus or minus 40 MHz to plus or minus 50 MHz. Therefore the transmission signal component in a local channel overlaps with a transmission signal component of an adjacent channel having a central frequency 80 MHz from that of the local channel. The bit error ratio of the adjacent channel is thus increased as in the example shown in FIG. 1. As a result, a plurality of channels of the system go down and in the case of a single supplementary line, that part of the system which goes down cannot be saved.
As described above, the major problem with the conventional relay receiver shown in FIG. 2 is that in the case of a decrease in the receiving level of the receiving signal, the bit error ratio of the channels relating to the receiving signal increases upon transmission of a digital signal. The result is interference between adjacent channels.
In order to solve the above problem, for example, consideration has been given to make the band width of intermediate frequency band pass filter 26 in FIG. 2 narrow enough only to allow the transmitted signal to pass. In the case shown in FIG. 3B, it may be considered that the intermediate frequency band pass filter 26 is made to pass only the frequency component of f.sub.IF plus or minus 25 MHz. However, a filter with a narrow band pass filter such as stated above degrades the amplitude delay characteristic of a passing signal every time the passing signal is made to pass through the filter. Therefore, if the characteristic of the above filter is made to have a uniformly narrow band width, the amplitude delay characteristic of the passing signal is degraded with an increase in the number of the relaying operations, and the bit error ratio of the channel relating to the passing signal is increased in accordance with the degradation in the amplitude delay characteristic of the passing signal.